The majority of the world's digital data is stored on a magnetic hard disk drive (HDD), which typically includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk, and an actuator arm that swings the suspension arm to place the read and/or write heads over selected circular tracks on the rotating disk. The slider body contains features on the media facing side (MFS) that create an air bearing that enables the slider to fly at a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic configurations to and reading magnetic signal fields from the rotating disk. The write and read heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The volume of information processing in the information age is increasing rapidly. Accordingly, an important and ongoing goal involves increasing the amount of information able to be stored in the limited area and volume of HDDs. Increasing the areal recording density capability (ADC) of HDDs provides one technical approach to achieve this goal. Methods to increasing the ADC of HDDs may involve improving components associated therewith (e.g., magnetic recording heads and/or magnetic media), improving system tolerances (e.g., servo systems), and improving data error correction schemes.
For example, one such method to increasing ADC involves reducing the size of the magnetic grains included within a magnetic recording layer. However, reducing the size of the magnetic grains may affect their thermal stability, thus leading to magnetization reversal and the loss of recorded data due to thermal fluctuations. The thermal stability of a magnetic grain is given by: KuV/kbT, where Ku denotes the magnetic anisotropy constant of the magnetic recording material. V is the grain volume, kb denotes the Boltzmann constant, and T denotes the temperature. Preferably, the average KuV/kbT needs to be larger than ˜60 to avoid thermal decay of media magnetization and loss of customer data. To compensate for the reduction in volume, V, of the magnetic grain, the magnetic anisotropy (Ku) thereof may be increased to maintain thermal stability. A problem with increasing the magnetic anisotropy of the magnetic recording material is the accompanying increase in the magnetic anisotropy (and thus the coercivity) of the magnetic recording material, which may exceed the switching field (i.e., the write field) capability of the write head.
Heat assisted magnetic recording (HAMR), also referred to as thermally assisted magnetic recording (TAR), has emerged as a promising magnetic recording technique to address the difficulty in maintaining both the thermal stability and write-ability of the magnetic media. As the coercivity of the magnetic recording material is temperature dependent, HAMR employs heat to lower and eliminate the coercivity of a localized region of the magnetic media and write data therein. The data state becomes stored, or “fixed.” upon cooling the magnetic media to well below the Curie temperature of the media (typically between 300° C. and 500° C.) in the applied field of the head. At normal drive operating temperatures, typically in a range between about 15° C. and 65° C.), the coercivity and KuV/kbT of the media is sufficiently high, that the full head write field and thermal agitation does not degrade the magnetic states. Heating the magnetic media may be accomplished by a number of techniques such as directing electromagnetic radiation (e.g. visible, infrared, ultraviolet light, etc.) onto the magnetic media surface via focused laser beams and near field optical antennas. HAMR techniques may be applied to longitudinal and/or perpendicular recording systems, although the highest density storage systems are more likely to be perpendicular recording systems.
As indicated above, HAMR allows use of magnetic recording materials with substantially higher magnetic anisotropy and smaller thermally stable grains as compared to conventional magnetic recording techniques. Moreover, an additional approach to increase ADC may involve storing multiple data bits per physical bit. However, such an approach is difficult to employ in perpendicular magnetic recording media. For instance, while the use of multiple stacked magnetic recording layers has been proposed, the secondary (tertiary, etc.) layers are farther from the magnetic head, leading to low signal-to-noise ratio (SNR) and difficulties associated with independently addressing bits in the different layers.